There are many applications where ultrashort laser pulses (i.e., laser pulses in the picosecond to femtosecond range) are required. Such applications include fundamental research in chemistry, physics, and biology involving time domain spectroscopy measurements and optical ranging and commercial applications including testing of optoelectronic devices, medical technology, optical communication and signal processing.
Heretofore, ultrashort laser pulses have been obtained utilizing two general techniques. These techniques are (a) active mode locking requiring time varying modulation of either laser gain or loss; and (b) passive mode locking involving the use of saturable absorbers, usually in the form of a dye, within the laser cavity. Active mode locking is disadvantageous in that it generally requires a relatively high degree of laser system complexity and cost. In such systems, either the pump laser must be constructed to produce short pulses, or an active modulator must be incorporated into the laser cavity. Thus, while active mode locking preserves laser tunability, this tunability is achieved at the expense of system complexity and cost. Cost is also increased due to the expensive, high stability, RF driver which is required for this approach.
Conversely, passive mode locking with a saturable absorber media typically has a spectral bandwidth that constrains the laser operating wavelength. While femtosecond pulses are easily generated in passive systems, the tunability of such systems is restricted to about 10-20 nm. The cost of maintaining and replacing saturable absorbers can also be relatively high, being comparable to that for actively mode-locked systems.
While active mode locking is capable of providing pulsed laser operation in the picosecond range, further processing of the laser output is normally required in order to achieve operation in the femtosecond range. One technique which has been employed to shorten laser output pulses and improve active mode locked performance is to provide an external cavity utilizing a nonlinear optical fiber. The output from the main laser cavity is interferometrically combined with a portion of the output beam which has been directed into the nonlinear optical fiber and retroreflected back to the main laser cavity. Pulse shortening is achieved by properly matching the length of the external cavity and the main cavity, such that interferometric addition of the pulse peak and interferometric cancellation of the pulse wings is obtained. However, all current systems utilizing nonlinear external cavities have demonstrated pulse shortening of a laser already operating as a short pulse laser. Thus, these lasers have all of the system complexity, cost and operating constraints discussed above in addition to the complexity and cost involved in pulse shortening through the use of the nonlinear external cavity.
A need therefore exists for a method and apparatus for generating an ultrashort pulsed laser output which is tunable, generally over the full tuning range of the laser utilized, without involving the high cost and complexity of current active mode-locked/coupled cavity systems. Basically, the system should be capable of utilizing a continuous wave pump signal to produce ultrashort laser output pulses in a single operation.